Quantum well infrared detector

ABSTRACT

The semiconductor component, comprises a succession of alternating stacked layers of a III-V semiconductor material with a large forbidden band such as Al x  Ga 1-x  As and a III-V semiconductor material with a small forbidden band such as GaAs with p-doping, defining a quantum (9) with sub-bands of HH and LH type in the region of the layer comprising the material with a small forbidden band in the valence band diagram (E v ) of each corresponding heterostructure. According to the invention, the thickness of the material with a small forbidden band is essentially selected in such a manner that only two quantum sub-levels LH 1  and HH 1  appear in the well, and the energy difference between these two sub-levels corresponds to the energy of the photons (6) to be detected, and the composition of the material with the large forbidden band is essentially selected in such a manner that the height adjacent the barrier (ΔE v ) of the quantum well is equal to or greater than the energy of the LH 1  sub-band.

This invention concerns semiconductor infrared detectors.

These detectors have particularly advantageous applications in the rangefrom 8 μm to 12 μm because these wavelengths correspond to a transparentwindow in the atmosphere. However, although this is a preferredsensitivity range, the invention is not limited to this particular rangeof values.

The material used most often for these semiconductor detectors is thealloy HgCdTe but this material is metallurgically extremely complex,which makes its industrial production difficult.

Several laboratories have recently proposed using a new type of infrareddetector--of the generic type to which the detector of the inventionpertains--based on the principle of absorption of photons associatedwith transitions between two quantum sub-bands appearing in the quantumwells created by stacking a very large number of alternating epitaxiallayers of III-V semiconductor material.

The advantage of these detectors comes mainly from the fact that the useof III-V compounds in place of II-VI compounds such as HgCdTe involvesmatellurgy that is much more manageable, making it easier to effectindustrial production of these detectors.

However, for various reasons which are explained in detail below, theperformance of these quantum well detectors with III-V semiconductorshas so far been inferior to that of HgCdTe detectors.

One of the objects of the invention is to overcome this limitation, byproviding a quantum well detector with III-V semiconductors which has ahigh detection sensitivity.

To this end, the detector of the invention, which is of the typespecified above, i.e. comprising a succession of alternating stackedlayers of a III-V semiconductor material with a large forbidden band anda III-V semiconductor material with a small forbidden band withp-doping, defining a quantum well with sub-bands of HH and LH type inthe region of the layer comprising the material with a small forbiddenband in the valence band diagram of each corresponding heterostructure,is characterized in that the thickness of the material with a smallforbidden band is essentially selected in such a manner that only twoquantum sub-levels LH₁ and HH₁ appear in the well, and in that theenergy difference between these two sub-levels corresponds to the energyof the photons to be detected, and the composition of the material withthe large forbidden band is essentially selected in such a manner thatthe height of the barrier adjacent the quantum well is equal to orgreater than the energy of the LH₁ sub-band.

As to the material with a large forbidden band, its thickness is veryadvantageously so selected that the potential barriers defined by thelayers of this material are sufficiently low for the resonant tunneleffect occurring through these barriers of light holes populating thesub-level LH₁ to create for these light holes a state in which the wavefunction thereof is spread in the assembly of the quantum wells andpotential barriers, while that of the heavy holes populating the HH₁sub-level is localized.

The material of this structure with a large forbidden band is preferablyAl_(x) Ga_(1-x) As and the material with a small forbidden band GaAs. Inthis case, the thickness of the material with a small forbidden bandlies in the range 1.5 nm to 2.5 nm approx., and the thickness of thematerial with a large forbidden band is about 8 nm approx. In a variant,the material with the small forbidden band can be In_(y) Ga_(1-y) As,with an indium content Y_(In)≦ 0.05 approx.

The invention is now explained in more detail, with reference to theaccompanying drawings. In all of the figures the same reference numeralsdesignate like parts.

FIG. 1 is a schematic representation of the conduction band of a stackof AlGaAs/GaAs layers.

FIGS. 2a and 2b show the behavior of the conduction band of a prior artstructure, respectively when quiescent and when biased, but with thethickness of the layers of GaAs reduced in such a manner that the energydifference of the quantum sub-bands corresponds to the wavelength of thelight to be detected.

FIGS. 3a and 3b correspond to FIGS. 2a and 2b for the case in which thecomposition of the AlGaAs layer has been so selected that the higherquantum sub-level comes level with the edge of the quantum well.

FIG. 4 is a schematic representation of the valence band of a stack ofAlGaAs/GaAs layers.

FIGS. 5a and 5b illustrate the behavior of the valence band of a priorart structure such as that of FIG. 4, respectively when quiescent andbiased.

FIGS. 6a and 6b correspond to FIGS. 5a and 5b but for a structure inwhich the doping and dimensions of the layers have been determined inaccordance with the teaching of the invention.

FIGS. 7a and 7b correspond to FIGS. 6a and 6b, for a modified embodimentof the invention.

The current state of quantum well detectors is firstly recalled,especially the mechanism whereby these detectors operate.

These detectors are essentially formed by a stack of heterostructures 1,as illustrated in FIG. 1, each formed by a layer 2 of GaAs and a layer 3of AlGaAs. These different layers are deposited epitaxially one on theother and the complete stack can comprise up to about fiftyheterostructures 1. This arrangement of layers creates a correspondingsuccession of discontinuities, alternately quantum wells 4 and potentialbarriers 5, in the conduction band E_(c), shown schematically in FIG. 1.

If the GaAs layers are thin enough, in the order of a few nanometers,quantum sub-bands (quantum levels) E₁, E₂, E₃, etc. appear from thequantum effect. By appropriate selection of the thickness of the GaAslayers, i.e. the width of the quantum wells 4, it is possible to adjustthe energy positions of the E₁ and E₂ sub-bands in such a manner thatthe difference ΔE₁ -ΔE₂ between the energies of the quantum levels E₂and E₁ will be in the order of 124 meV approx., as illustrated in FIG.2a. If such a structure is illuminated by light with a wavelength of 10μm, i.e. by photons with energy h=124 meV, these photons will induce aresonant electronic transition from the sub-band E₁ to the sub-band E₂,symbolized by the arrow 7. If an electric field is applied to such astructure (FIG. 2b), that is to say if the component is biased, theelectrons in the sub-band E₂ can pass by the tunnel effect through thepotential barriers 5 corresponding to the AlGaAs layers, as symbolizedby the arrow 8, thus generating a measurable photo-current.

The first detectors implemented on this principle have been described byB. F. Levine et al., New 10 μm Infrared Detector Using IntersubbandAbsorption in Resonant Tunneling GaA1As Superlattices, Appl. Phys.Lett., Vol. 50, No. 16, p. 1092 (1987). Account is taken there of theincidence of a parasitic tunnel current due to electrons in level E₁whose tunneling transparency, although less than that of the electronsin level E₂ cannot be neglected. This tunnel current thus creates a highdark current which is detrimental to the performance of the detector.

In this respect, it is known that the tunneling transparency isexpressed by an equation of the type:

    T=A exp[-(m*.sup.1/2 ΔE.sup.3/2 d)/V],               (1)

where:

m* is the mass of an electron,

ΔE is the height of the barrier for the electron in question (ΔE₁ forthe sub-band E₁ and ΔE₂ for the sub-band E₂),

d is the thickness of the potential barrier of AlGaAs, and

V is the applied voltage.

It has then been proposed to increase the thickness d of the barrierand/or to increase its height ΔE₁. However, doing this also reduces thetunneling transparency of the electrons in the sub-band E₂, i.e. thosewhich generate the photo-current. A compromise appears to be difficultto achieve and the quantum well detectors with the best performancecurrently made are designed in such a manner that the E₂ sub-band islocated just at the edge of the well, as seen in FIGS. 3a and 3b,

It is thus possible to increase the thickness of the potential barrier 5or in other words reduce the dark current without however affecting thephoto-current. Reference may be made in this respect to the work of B.F. Levine et al., Bound-to-Extended State Absorption GaAs SuperlatticeTransport Infrared Detectors, J. Appl. Phys., Vol. 64, No. 3, p. 1591(1988), which refers to a "detectivity" D*=10¹⁰ cm. Hz^(-1/2) /W at 77K., a result close to but still less than that obtained with aconventional HgCdTe detector, which yields 3×10¹⁰ cm. Hz^(-1/2) /W atthe same temperature.

A first limitation in the performance of this structure is the lowlifetime of the electrons in the structure, which is related to thelifetime of electrons in the material (AlGaAs) forming the potentialbarrier in which the hot electrons move.

A second limitation is similar to the phenomenon of inter-sub-bandoptical absorption which should obey certain rules of quantum mechanicalselection prohibiting light incident perpendicular to the plane of thelayers.

Very recently, this second difficulty has been tackled by B. F. Levineet al., Normal Incidence Hole Intersubband Absorption Long WavelengthGaAs/Al_(x) Ga_(1-x) As Quantum Well Infrared Photodetectors, Appl.Phys. Lett., Vol. 59, No. 15, p. 1864 (1991), which proposes to usequantum wells created by discontinuities in the valence band, not in theconduction band.

Thus, in the latter case, the optical inter-sub-band absorption can beobtained with light incident perpendicular to the plane of the layers.The cited article thus refers to a detection sensitivity of 3.1×10¹⁰ cm.Hz^(-1/2) /W at 77 K. for the wavelength 7.9 μm.

However, this proposal also suffers from limitations, due on thisoccasion to the low lifetime of the holes.

Thus, if we consider, as is illustrated in FIG. 4, the discontinuitiescreated in the valence band of the structure, which alternates quantumwells 9 with potential barriers 10, it is apparent that in each quantumwell 9 there are many sub-bands HH₁, HH₂, LH₁, etc., (the designation HHreferring to the heavy holes and LH to the light holes), Three of thesesub-bands are illustrated in FIG. 4 but the situation can vary veryconsiderably, depending on the width of the quantum wells 9 or theheight ΔE_(v) of the potential barrier 10.

FIGS. 5a and 5b represent schematically the case described by Levine inthe article last cited; it relates to quantum wells 9 which are 3 nm to4 nm wide with a barrier height ΔE_(v) in the order of 160 meV, for analuminum content of 0.30 of the material AlGaAs. (Here and in thefollowing the "content" is understood to be the mole fraction x_(Al) ofAl_(x) Ga_(1-x) As). In this case, the sub-band HH₂ is nearly at theedge of the well and the difference in energy between the edge of thewell and the sub-band HH₁ is 144 meV for a well of 3 nm or 157 meV for awell of 4 nm (FIG. 5a).

Under illumination (FIG. 5b), the photons with corresponding energy (144meV and 157 meV respectively) are absorbed (arrow 11), thus causingmovement of the holes in the continuum of the valence band (arrow 12)when an electric field is applied.

Since it is known that the effective mass of holes in the continuum ofthe valence band of AlGaAs (and GaAs) is very high, in the order of 0.4m_(o) (m_(o) being the mass of the electron), it will be understood thattheir mobility will be reduced and that their mean free path will beshort, in other words, their lifetime will be very short.

This prior art structure has another problem in the presence of thesub-band LH₁ in the quantum wells 9. Thus this sub-band is populatedwith holes of low effective mass which are about 50 meV from the edge ofthe quantum well. Considering equation (1) above, the tunnelingtransparency of these light holes is thus high, the more so because theFermi level approaches the level LH₁, i.e. the layer of GaAs isp-doped--which applies in this case with the doping lying between 10¹⁷cm⁻³ and 5×10¹⁸ cm⁻³ approx. Once again an irreconcilable compromise isencountered between a large barrier width, necessary to reduce the darkcurrent which would otherwise be very high, and a concomitant reductionof the photo-current and thus of the sensitivity of the detector.

The invention, which will now be described, specifically seeks toovercome this problem. Its teaching lies essentially in an appropriatechoice of the composition of the AlGaAs alloy and of the thicknesses ofthe layers of GaAs and AlGaAs.

The invention in particular used the phenomenon of inter-sub-bandtransition between HH₁ and LH₁ in the first place to eliminate theparasitic tunnel effect due to the light LH₁ holes. In the second place,once this parasitic effect has been eliminated, it is possible to reducethe width of the AlGaAs barrier insofar as this is needed, because theholes populating the sub-level HH₁ are very heavy, exhibiting a lowtunneling transparency and therefore only having a small effect on theperformance of the detector.

If equation (1) is applied, this reduction of thickness can be by afactor at least equivalent to:

    (m*.sub.HH1 /m*.sub.LH1).sup.1/2 =2.4 approx.

and at most equivalent to

    (m*.sub.HH1 /m*.sub.LH1).sup.1/2 (ΔE.sub.HH1 /ΔE.sub.LH1)3/2=12 approx.

where m*_(HH1) and m*_(LH1) are taken to be 0.4 m_(o) and 0.07 m_(o)respectively (m_(o) being the mass of the electron), and ΔE_(HH1) andΔE_(LH1) are the heights of the respective barriers associated with thesub-bands HH₁ and LH₁, with ΔE_(HH1) =190 meV approx. and ΔE_(LH1) =66meV approx.

For the detector to operate at the selected wavelength of about 10 μm itis necessary that the energy between the two sub-bands LH₁ and HH₁ shallbe in the order of 124 meV. It can be shown by quantum mechanicscalculations that, in this case, the width of the hole should be in theorder of 1.5 nm to 2.5 nm, this thickness depending on the height of thepotential barrier, i.e. on the aluminum content in the AlGaAs. Thecalculations show that the height of the barrier should exceed about 230meV, which requires an aluminum content greater than about 0.42.

A structure in accordance with the invention is illustratedschematically in FIG. 6a (at equilibrium) and FIG. 6b (underillumination and with an applied electric field).

The quantum well detector of the invention is thus formed from asuccession of quantum wells 9 of very small width (around 1.5 nm to 2.5nm), separated by potential barriers 10 which are themselves relativelythin.

It is known that the LH₁ holes, which have a low effective mass, canpass through thin barriers by the tunnel effect, even if the height ofthe barrier ΔE_(v) is above their energy level, and this the more easilywhen these levels present the same energy, thus creating the well-known"resonant tunnel effect". The holes populating the sub-band LH₁ thuscome to move freely, perpendicular to the plane of the layers, in aperiodic potential, sometimes also known as a "superlattice".

It is thus possible to be released from too precise a definition of thethickness of the AlGaAs by using barrier heights such that the sub-bandLH₁ is located right at the edge of the quantum well, as shown in FIGS.7a and 7b; in this case, the height ΔE_(v) of the barrier adjacent thequantum well is no longer greater than but just equal to the energy ofthe sub-band LH₁. This configuration is close to that shown in FIGS. 5aand 5b (corresponding to the proposal of Levine in the cited article)but with the essential difference of the presence, in the prior artsituation, of the LH₁ sub-band located in the quantum well below theenergy of the potential barrier, contributing thus to an increase in thedark current.

It is also known that although the sub-band LH₁ is quantized, it has itswave function completely spread over the whole structure, by theresonant tunnel effect, which leads to an increase δE_(LH1) in theenergy of the sub-band, as can be seen at 13 in FIGS. 6a and 6b. Thisincrease δE_(LH1) depends on the width of the well. For wells 2 nm wide,the increase is in the order of 50 meV, 15 meV and 5 meV for barriers 5nm, 7.5 nm and 10 nm wide respectively (the height of the barrier alsohaving an effect on these values).

As for the holes populating the sub-band HH₁, which do not only have alarge effective mass but which are also located deep in the hole, thesedo not experience the tunnel effect and their wave function also remainslocalized.

In summary, the light holes LH₁ moving in the periodic potential have alifetime longer than that of the heavy holes moving in the continuum ofthe valence band, as explained with reference to FIG. 5.

Moreover, although the movement of holes in the band LH₁ will be favoredby a small barrier width (a small thickness of the AlGaAs layer), it isdesirable not to reduce this value, because too large an increase inΔE_(LH1) will result in a reduction in the energy difference between thesub-bands HH₁ and LH₁, so increasing the dark current.

Knowing however that infrared detectors operate at the temperature ofliquid nitrogen, namely 77 K., and that the thermal energy is 6.6 meV atthis temperature, it is only important that the increase ΔE_(LH1) shallbe less than 6.6 meV. It is thus possible to find an optimum barrierwidth, defined in such a manner that ΔE_(LH1) =6.6 meV approx. For wells2 nm wide, this optimum corresponds to a thickness of AlGaAs of about 8nm (a value which can vary with the aluminum content of the AlGaAs).

Taking into account the very low values of the width of the quantumwells which have been indicated, a variation of an atomic monolayer,i.e. in the order of 0.2 nm to 0.3 nm involves an relative variation inthickness of 10%. It is thus necessary to use techniques such asmolecular beam expitaxy (MBE) or metal organic chemical vapor deposition(MOCVD) which are compatible with such precision and ensure gooduniformity of the epitaxy over the whole surface of the semiconductorchip.

Moreover, the invention is not limited to a GaAs/A1GaAs heterojunctionand its teaching can apply equally to other heterostructures formed onthe basis of III-V alloys.

In particular the alloy InGaAs can be used in place of GaAs to realizethe quantum wells. In particular, this alloy InGaAs having a latticeconstant greater than that of GaAs, it will be subjected to a uniaxialstrain which will have the effect of pushing the sub-band LH₁ furtheraway in energy, in other words of increasing the energy differencebetween the sub-bands HH₁ and LH₁. It is possible, for the sameperformance, to enlarge the quantum well by this phenomenon, relative toa similar structure using GaAs, and thus to facilitate practicalimplementation by virtue of a greater thickness of the layer to bedeposited.

This effect is particularly marked: thus, for an indium content (i.e. amole fraction Y_(In) of In_(y) Ga_(1-y) As) of only 0.03, thus for acomposition very close to that of GaAs, the increase in the energy is inthe order of 15 meV. It is however noted that alloys with an indiumcontent greater than 0.05 cannot really be used, because the excessivestrain then risks creation of interfacial dislocations, given the totalthickness of the quantum well structure.

I claim:
 1. A semiconductor component of quantum well infrared detectortype comprising a succession of alternating stacked layers of III-Vsemiconductor material with a large forbidden band and a III-Vsemiconductor material with a small forbidden band with p-doping,defining an assembly of potential barriers alternating in series withquantum wells, each well including sub-bands of HH and LH type in theregion of the layer comprising the material with the small forbiddenband in the valance diagram (E_(v)) of each correspondingheterostructure, characterized in that:the thickness of the layers ofthe material with a small forbidden band is selected such that only twoquantum sub-levels LH₁ and HH₁ appear in each well, and in that theenergy difference between the two sub-levels corresponds to the energyof photons to be detected, and the material with the large forbiddenband comprises a composition that causes the height (ΔE_(v)) of thepotential barriers to be equal to or greater than the energy of the LH₁sub-band.
 2. The component of claim 2 wherein, forresonant-tunnel-effect light holes populate the sub-level LH₁ andresonant-tunnel-effect heavy holes populate the HH₁ sub-level, thethickness of the layers of the material with a large forbidden band isso selected that the height of the potential barriers defined by thelayers of said material creates for the light holes a state in whichlight-hole wave function is spread over the assembly, while heavy-holewave function is localized.
 3. The component of claim 1 wherein thematerial with a large forbidden band is Al_(x) Ga_(1-x) As.
 4. Thecomponent of claim 3 wherein the material with a small forbidden band isGaAs.
 5. The component of claim 4 wherein the thickness of the layer ofthe material with a small forbidden band lies approximately between 1.5nm and 2.5 nm.
 6. The component of claim 4 wherein the thickness of thelayers of the material with a large forbidden band is approximately 8nm.
 7. The component of claim 3 wherein the material with a smallforbidden band is In_(y) Ga_(1-y) As with an indium content ofapproximately Y_(In) ≦0.05.